How to land a Mini on Mars

The biggest and fanciest Mars rover so far will soon blast off from Florida

EARTH excepted, the most probed and prodded planet in the solar system is Mars. Besides the assortment of craft that have flown by it or gone into orbit around it, three robotic buggies equipped with cameras and scientific instruments have roamed the Martian surface on behalf of NASA, America's space agency, since 1997.

If all goes according to plan, they will soon be joined by a fourth. On November 26th a new rover, Curiosity, will ascend from Cape Canaveral. If it gets there in one piece, it will examine the climate and geology of Mars and look for any signs of life that might have arisen.

The first of NASA's rovers, Sojourner, which reached Mars in 1997, was 65cm long and weighed (on Earth, where the gravitational pull is 2½ times Mars's) 10kg. Spirit and Opportunity, its twin successors, were larger, at 1.6 metres and 170kg. Curiosity, by comparison, is a monster. At 3 metres and 900kg it is the size of a small car. It also uses different technology. The other three rovers were powered by solar panels. Curiosity is powered by plutonium. (Not a full-scale reactor, but a generator that turns the heat of radioactive decay into electrical energy.) This brings three advantages. First, it allows Curiosity to carry more power-hungry scientific instruments than previous rovers. Second, it permits the rover to work through the Martian winter. Third, it avoids the problem of dust accumulating on the solar panels, which gradually sapped the strength of its predecessors.

Curiosity's size makes getting it safely onto the Martian surface tricky. Previous rovers have deployed parachutes to slow their descents, and have then crashed into the ground using airbags to cushion their impacts. Curiosity is too massive for that approach to work. Instead, NASA hopes to deposit it on Mars using a contraption it has dubbed a skycrane.

As with the other rovers, Curiosity's mother ship will rely on heat shields and air-resistance, and then on a parachute, to slow its arrival. But at an altitude of 1.6km a specially designed descent stage bearing the rover will drop away from this vehicle. The descent stage has eight rocket motors on its corners. These will slow its fall to a relatively sedate 0.75 metres a second. When it is about 20 metres above the surface, the rover will be lowered from it on wires and deposited gently onto the Martian landscape. The cables will then be cut with explosives, the descent stage will fly off and crash land elsewhere, and Curiosity will begin its mission.

That, at least, is the theory. But the skycrane has never been used before, and there is plenty else that could go wrong. Indeed, Mars has something of a reputation for destroying spacecraft. Around half the missions sent there since the first Soviet attempts in 1960 have failed to arrive. A conversation on the subject in 1964, between a journalist and John Casani, a NASA scientist, spawned the idea of a Great Galactic Ghoul, a malevolent creature that prowls the space-lanes between Earth and Mars, dining on unfortunate spacecraft.

The ghoul's latest victim appears to have been Phobos-Grunt, an ambitious Russian mission that was intended to return to Earth with a rock sample from Phobos, the larger of Mars's moons. The Russian space agency's engineers lost contact with it soon after its launch on November 8th. Limited contact had been re-established as The Economist went to press, but it is not clear whether the mission can be salvaged. NASA's engineers, rationalists though they be, will be keeping their fingers crossed on Saturday, and hoping that the ghoul's appetite has thus been sated, and that it will leave Curiosity alone.